Field
The present disclosure relates to a radiation detector that corrects detection signals of annihilation radiation, and more particularly to a radiation detector that converts radiation into fluorescence to measure the fluorescence and is capable of eliminating the influence of afterglow of the fluorescence by correction. The present disclosure also relates to a method of detecting radiation.
Description of Related Art
A specific configuration of a conventional positron emission tomography (PET) device, which images radiopharmaceutical distributions, will be described. A conventional PET device is provided with a detector ring. The detector ring includes radiation detectors which detect radiation and are arranged in an annular form. The detection ring detects a pair of radiation rays (annihilation radiation) emitted from a radiopharmaceutical inside a subject, the radiation rays travelling opposite directions.
The configuration of a radiation detector 51 will be described. As shown in FIG. 11, the radiation detector 51 is provided with a scintillator 52, in which scintillator crystals are three-dimensionally arrayed, and a photodetector 53, which detects fluorescence emitted from radiation absorbed in the scintillator 52. The photodetector 53 has a detection surface in which a large number of photodetection elements are arrayed in a matrix. Further, the detection surface of the photodetector 53 and one surface of the scintillator 52 are optically connected to each other.
When radiation enters the scintillator 52, fluorescence is generated inside the scintillator 52. It takes time for fluorescence to be completely attenuated. Therefore, when radiation enters the scintillator 52, the scintillator 52 continues to emit weak light for a while.
Therefore, in some cases, radiation may enter the scintillator 52 before the light emission of the scintillator 52 sufficiently stops. Accordingly, as shown in FIG. 12, light emissions from the scintillator 52 are detected in an overlapped state. Such a phenomenon is called fluorescence pileup. When pileup occurs, the radiation detector 51 cannot detect radiation correctly.
In view of the above, conventionally, countermeasures against pileup have been taken. For example, in a first method, a baseline is sequentially changed to detect piled-up fluorescence (see U.S. Pat. No. 6,903,344). Further, in a second method, piled-up fluorescence are separated into two detection signals by an estimation arithmetic operation (see M. D. Haselman et. al. “FPGA-Based Pulse Pileup Correction”, NSS/MIC record Nov. 13, 2010).
However, conventional radiation detection has the following drawback, which make it difficult to sufficiently correct fluorescence pileup.
First, a method in which the baseline is sequentially changed may not have particular drawbacks when there is a sufficient time interval in the generation of fluorescence as shown in FIG. 13. However, when the generation of prior fluorescence and the generation of subsequent fluorescence are close to each other in time as shown in FIG. 12, the subsequent fluorescence starts glowing while the prior fluorescence is being largely attenuated. In the first method, the baseline for the subsequent fluorescence is fixed at a state immediately before the subsequent fluorescence starts glowing. However, an actual baseline largely varies in the negative direction as indicated by an arrow of FIG. 12 while the subsequent fluorescence is glowing. In this manner, in the first method, fluorescence pileup is corrected not on the basis of a correct baseline. Specifically, the subsequent fluorescence is excessively baseline-corrected, and therefore overestimated.
Further, in the second method in which an estimation arithmetic operation of piled-up fluorescence is performed, the inclination of attenuation of the prior fluorescence is calculated to correct a detection result of the subsequent fluorescence. However, an attenuation state of fluorescence cannot be represented by a simple function such as a linear function. Therefore, in order to accurately separate the piled-up fluorescence, it may be necessary to fit a complicated function to the detection result of fluorescence. Performing such an operation every time pileup occurs is difficult to achieve because a huge amount of calculations is typically required.